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Carbon Dioxide Mediated C(sp3)–H Arylation of Amine Substrates Mohit Kapoor, Daniel Liu, and Michael C. Young J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 22 May 2018 Downloaded from http://pubs.acs.org on May 22, 2018
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Mohit Kapoor, Daniel Liu, Michael C. Young* Department of Chemistry and Biochemistry, School of Green Chemistry and Engineering, University of Toledo, Toledo, OH, 43606. Supporting Information Placeholder ABSTRACT: Elaborating amines via C–H functionalization has been an important area of research over the last decade, but has generally relied on an added directing group or sterically hindered amine approach. Since free amine-directed C(sp3)–H activation is still primarily limited to cyclization reactions, and to improve the sustainability and reaction scope of amine-based C–H activation, we present a strategy using CO2 in the form of dry ice that facilitates intermolecular C‒H arylation. This methodology has been used to enable an operationally simple procedure whereby 1º and 2º aliphatic amines can be arylated selectively at their γ-C–H positions. In addition to potentially serving as a directing group, CO2 has also been demonstrated to curtail the oxidation of sensitive amine substrates.
Amines are a key functional group in pharmaceuticals,1 agrochemicals (Figure 1),2 as well as materials,3 and have been the subject of numerous synthetic approaches.4 Although transition metalcatalyzed C–H activation has revolutionized the installation of functional groups at otherwise inert C–H bonds,5 amines are still a complicated substrate class for this strategy because of their reactivity. Primary (1º) and secondary (2º) amines are especially sensitive to oxidation, making them a challenge for organometallic reactions:6 palladium, for example, is well-known in its ability to oxidize 1º and 2º amines.7 Furthermore, amines can react with organometallics to produce substituted amines, such as in the BuchwaldHartwig coupling.8 Despite these challenges, there are examples of palladium-catalyzed C–H activation of both 1º and 2º free-amine substrates in the literature, yet these have generally featured either activation of more reactive C(sp2)–H bonds,9 or intramolecular cyclization.10 We present herein an alternative strategy for overcoming these barriers using carbon dioxide to mediate C‒H activation.
Figure 1. Biologically-Active Compounds Bearing a γ-Arylamine Motif. Recent work has suggested that in situ protection of the amine as an ammonium can be used to protect it from oxidation, thereby facilitating remote C(sp3)–H oxidation reactions by deactivation,11
while a recent report even suggests 1º amine or ammonium-directed C(sp3)-H oxidation.12 1º and 2º amines, however, generally require the presence of a directing group (DG) to facilitate C–C bond forming C(sp3)-H activation.13 The only exception to this is when 2º amine substrates are used bearing fully substituted α,α’centers.14 The strategy can also be applied to primary β-aminoalcohols by first converting them into highly substituted aminoketals, although neither strategy enjoys a particularly broad substrate scope.15 The two traditional methods to facilitate DG-mediated C(sp3)–H arylation of amines have been to use static functional groups, such as amides with or without a second chelating moiety (Scheme 1b),16 while a recent trend has focused on transient DGs (Scheme 1c).17 Though each can be used to facilitate C(sp3)–H activation reactions, both possess disadvantages that might be alleviated. Static DGs require additional stoichiometric reagents, and are both atom and step uneconomical. Meanwhile, transient DGs, generally based on aldehyde-based DGs, suffer from the presence of oxidation-sensitive imines as intermediates. Furthermore, static DGs are rarely used for C–H activation of 2º amine substrates, while transient DG methods are non-existent.
Scheme 1. Strategies for Amine-Directed C(sp3)–H Arylation. To circumvent the challenges of these two strategies, we sought a hybrid DG strategy (Scheme 1d). Carbon dioxide (CO2) was subsequently identified as a viable candidate: it contains a carbonyl that can reversibly react with amines, while the carbamate products can readily coordinate to palladium but are more chemically robust than imines.18 CO2 has previously been used as a traceless DG for the meta-C–H activation of phenols (although requiring separate and harsh installation and removal steps).19 We reasoned that CO2 could therefore serve as a transient DG in the presence of a suitable nucleophile, such as an amine – both serving as a protecting group, preventing substrate oxidation, as well as directing site selective C– H bond scission by Pd. The use of a transient carbamate moiety as a DG would have the potential to make C–H activation of 1º amines feasible under more oxidizing conditions where simple ammonium formation is inadequate to prevent amine oxidation, while simultaneously allowing expansion of a transient DG approach to 2º amines. We began our studies using the model substrate tert-amyl amine. Previous work17 had shown that in the absence of a DG, only trace
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C–H arylation of this substrate is observed. Gratifyingly, using conditions similar to Ge’s,17a we were able to selectively γ-arylate tertamyl amine using phenyl iodide in acetic acid with silver trifluoroacetate as an additive simply by performing the reaction under CO2 pressure (Table 1, 1a). As expected, the absence of any of these components leads to only trace or no product. While the reaction can occur under 1 atm of CO2 (see SI), we determined that the reactions could be easily screened in standard reaction vials by adding CO2 in the form of dry ice. Approximately one-to-two equivalents of dry ice could be added at 0.3 mmol scale, and the vials immediately sealed to achieve satisfactory pressure to promote the desired C‒H activation without concomitant failure of the vials (see SI for further details and notes on safety). Interestingly, no exogenous ligand was required to facilitate this transformation.
donating substituents (1r – 1w), as well as iodides with extended conjugation (1x and 1y) are also tolerated in the reaction. While use of ortho-functional groups other than a chelating ester were not tolerated using the standard conditions, switching the silver additive to AgOTf allowed substituents with ortho-fluoro (1z) and hydroxyl (1aa) groups to participate in the reaction. The reason for this modulation of reactivity is not clear at the present time, although the reaction using AgOTf is more acidic than the AgTFA reaction by ~1 pH unit.
Table 2. 1º Amine Substrate Scope for γ-C(sp3)–H Arylation Directed by CO2. a 90ºC. b From isolated Bz products. c Extra AcOH molecule removed for clarity.
Table 1. Aryl Iodide Substrate Scope for γ-C(sp3)–H Arylation of 1º Amines Directed by CO2. a AgOTf used in place of AgTFA. With the optimized conditions in hand, we first explored the range of aryl iodides that could participate in the reaction. In general, electron poor aryl iodides containing fluorides, trifluoromethyls, and esters were all effective in the reaction (Table 1, 1b – 1l). While ortho-substituents are often challenging in C–H activation reactions, the presence of an ortho-ester is well tolerated, likely due to a chelate effect that promotes rather than inhibits oxidative addition to the C–I bond.20 Halophenyl iodides are also tolerated (1m – 1o), with no sign of activation of either aryl chlorides or bromides. Even a diiodide gives good selectivity for monofunctionalization (1o), while both halides can be functionalized in the presence of excess amine (see SI). More complex heterocycles can also participate in the reaction (1p and 1q), although with somewhat limited success due we suspect to partial decomposition of the heterocycles during the reaction. Iodides with moderate and strongly electron
Considering the amine scope, substrates containing saturated (2a – 2d) and unsaturated (2e) carbocycles all participate in the reaction with relatively good yields. The precursor for 2d was an inseparable mixture of cis (major) and trans (minor) isomers, yet we were able to cleanly isolated the cis isomer of 2d after the reaction (the minor trans isomer also gave product, although this could not be cleanly isolated). While 2e has more reactive γ-C(sp2)–H bonds, good selectivity was observed for arylation of only the C(sp3)–H bond. This selectivity can be rationalized due to the inflexibility of the fused rings, which likely prevents the more reactive C–H bonds from accessing the coordinated palladium catalyst. As long as a single type of γ-C–H bond is present on the substrate, good selectivity could be achieved despite the length of other aliphatic chains (2f and 2g). When there were symmetrical positions with γ-C–H bonds, however, a mixture of mono and diarylation was observed (2h and 2i), albeit with some selectivity for mono and diarylation only – triarylation was not observed when three separate ethyl groups were attached (2i). We found that the 1º amine substrates did not require an α-tertiary center, and less substituted substrates could also participate in the reaction simply by lowering the temperature (2j – 2m). Despite symmetrical γ-C–H bonds in both 2k and 2m, only monoarylation was observed. The enhanced selectivity for monoarylation (compared to 2h and 2i) is likely due to the decreased reaction temperature. Despite the presence of a chelating ester moiety, the ethyl ester
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Journal of the American Chemical Society of valine could be selectively arylated once to give the unnatural amino acid product (2n). This compound was functionalized with retention of the (S)-stereocenter of valine, and with moderate diastereoselectivity (see SI). We were delighted to find that using a rigid bornylamine in which there is a 1º γ-C–H bond that cannot be reached by the catalyst, selective transannular arylation at a methylene γ-C–H bond position was achieved instead of at the methyl group (2n).21 Using a larger biphenyl iodide, a similar product could be obtained (2m), which facilitated x-ray analysis to confirm location of the aryl group. We next turned our attention to the challenge of whether or not CO2 could also serve to promote the γ-C(sp3)–H arylation of 2º amines, a transformation that had required highly substituted substrates up to this point, generally gave β rather than γ-functionalization, and which was inaccessible via imine-type DGs. Gratifyingly, the reaction could be performed by modifying our conditions, most notably by increasing the CO2 loading. The excess CO2 was critical – at lower concentrations not only was the product yield decreased, but significant oxidation of the starting material was observed, giving a mixture of imine, amine, and aldehyde.
Table 3. 2º Amine Substrate Scope for γ-C(sp3)–H Arylation Directed by CO2. The reaction tolerates 2º amines with different length alkyl substituents on the nitrogen (Table 3, 3a – 3c). Although 3a possesses two distinct terminal γ-C–H bonds, the reaction occurs selectively on the more substituted side, presumably due to more favorable comformation during the C–H activation step.10a A homobenzylic 2º amine substrate was also tolerated without concomitant oxidation (3d), as were a variety of benzylic amines, all with complete selectivity observed for the γ-C(sp3)–H bond rather than functionalization on the arene (3e – 3k). These are interesting substrates both from the perspective of potentially competitive C(sp2)‒H bonds, as well as to the sensitivity to oxidation. Again, the putative more favored conformation during the reaction facilitates complete selectivity for the less reactive C–H bonds, even though the increased flexibility was initially predicted to lead to γ-C(sp2)–H functionalization (2f). It is noteworthy that negligible oxidation of the benzylic amines to the corresponding imines was observed in these reactions, with significant mass balance being the unreacted amine. Regrettably, increasing catalyst loading or adding the catalyst portionwise failed to drive these reactions to completion, a result which demands further scrutiny to understand. Less substituted 2º amine substrates bearing a more oxidatively-sensitive α-2º center could also be utilized (3l and 3m). Finally, we wanted to explore the mechanism and role of CO2 in the reaction. Salt 4 was prepared and then subjected to the reaction conditions without additional CO2, and gave greater than stoichiometric conversion with respect to CO2, suggesting that CO2 is acting transiently (Figure 2A). When the reaction was performed in
AcOD, no deuteration was observed, suggesting that the concerted metalation-deprotonation step may be irreversible (Figure 2B).22 This was further corroborated by KIE experiments, which showed a significantly faster reaction for the proteo-substrate.
Figure 2. Mechanistic Investigations of the CO2-Mediated γC(sp3)–H Activation of Aliphatic Amine. Although Pd–carbamato complexes are known,23 it is possible that CO2 could actually have an off-cycle roll in this reaction. To investigate whether or not CO2 was actually serving to disperse catalytically inactive Pd-diamine complexes, we prepared complex 5 (Figure 2C).24 Dissolution of 5 in AcOH-d4 without CO2 was satisfactory to partially dissociate the complex to give a mixture of the trinuclear complex 6 and free ammonium, suggesting CO2 isn’t necessary under the reaction conditions to disrupt formation of complex 5. Meanwhile, introduction of CO2 into CDCl3 solutions of this complex gave no change –extra base is required to facilitate proton transfer to lead to stable Pd–carbamate complexes.23 To simulate the reaction conditions, PdCl2 and amine were mixed in 1% AcOH in DMSO-d6, followed by bubbling CO2 through the solution for 12 h. This gave a complex spectrum containing a carbamate signal, as well as new resonances between 121-124 ppm in the 13C NMR from different CO2 species. We believe this supports an on-cycle role for CO2 as a transient DG acting through a rare 7membered palladacycle,25 though further studies are needed. In conclusion, we have described the first example of a CO2 mediated amine C–H activation. The ability of CO2 to transiently form carbamates may be useful not only for C–H activation, but also other directing group mediated reactions. Furthermore, we anticipate that use of CO2, rather than a traditional protecting group, may be a viable strategy for improving the sustainability of organic synthesis. Work is underway in our lab to better understand the mechanism and intermediates at play in these reactions, as well as to further develop the scope of these transformations.
The Supporting Information is available free of charge on the ACS Publications website.
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Prof. Michael C. Young,
[email protected].
The authors wish to declare that this chemistry is the subject of a provisional patent (United States Provisional Patent #62/608,074).
The authors wish to acknowledge start-up funding from the University of Toledo, as well as a grant from the ACS Herman Frasch Foundation in partial support of this work. Ms. T. Perera and Ms. K. Rajanayake are acknowledged for collecting high resolution ESI-MS data at The University of Toledo, as well as Dr. K. Suhr at The University of Texas at Austin MS Core Facility.
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